I don't know how we don't have a post talking about the livestream where they (briefly) discussed the new Difficulty, but mainly the Expeditions. We don't have extensive text explaining it, but the dev-stream gave us the details
The cliffnotes version: Expeditions will be the default raiding process, replacing the current raiding screen. They are where each tier of difficulty has outposts set in paths, and each step on the path gives you a particular resource for completing it. Here's a pic of the Expedition screen. There are multiple paths so you choose which thing to pursue. Some of those rewards are synth/parts/etc that we're used to, but more have Enhancements.
An enhancement is a raid-long gameplay boost. Something like "Increase your grapple range" or "jump 3 times instead of 2" or "extra ammo" etc. Each type has multiple tiers (so how powerful the boost and how frequently you get it). You can go into a raid with 3 enhancements. After the raid, the enhancements are lost.
When you complete an entire expedition, you get a whole set of rewards. Exclusive cosmetics, a decal, currency reward, rarer enhancements, and mystery reward/random one. This isn't a battle pass, you don't pay to jump ahead, it's only rewarded for doing the expedition. When you finish the expedition you go to the next expedition.